Manufacturing method of semiconductor device

ABSTRACT

One object is to have stable electrical characteristics and high reliability and to manufacture a semiconductor device including a semi-conductive oxide film. Film formation is performed by a sputtering method using a target in which gallium oxide is added to a material that is easy to volatilize compared to gallium when the material is heated at 400° C. to 700° C. like zinc, and a formed film is heated at 400° C. to 700° C., whereby the added material is segregated in the vicinity of a surface of the film and the oxide is crystallized. Further, a semi-conductive oxide film is deposited thereover, whereby a semi-conductive oxide having a crystal which succeeds a crystal structure of the oxide that is crystallized by heat treatment is formed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device including asemi-conductive oxide and a manufacturing method thereof.

In this specification, a semiconductor device generally means a devicewhich can function by utilizing semiconductor characteristics, and anelectro-optical device, a semiconductor circuit, and electronic devicesare all semiconductor devices.

2. Description of the Related Art

In recent years, a technique for forming transistors using asemiconductor thin film (with a thickness of approximately several tensto several hundreds of nanometers) formed over a substrate having aninsulating surface has attracted attention. Transistors are applied to awide range of electronic devices like integrated circuits andelectro-optical devices, and transistors that are to be used asswitching elements in image display devices, in particular, has beendeveloped.

Some oxides have semiconductor characteristics. Examples of such oxideshaving semiconductor characteristics are a tungsten oxide, a tin oxide,and an indium-gallium-zinc-based oxide (In—Ga—Zn-based oxide), and athin film transistor in which such an oxide having semiconductorcharacteristics is used for a channel formation region is known (seePatent Documents 1 and 2). Further, in particular, properties of anIn—Ga—Zn-based oxide also have been researched (Non-Patent Document 1).

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-96055

Non-Patent Document

-   [Non-Patent Document 1]-   Toshio Kamiya, Kenji Nomura, and Hideo Hosono, “Origins of High    Mobility and Low Operation Voltage of Amorphous Oxide TFTs:    Electronic Structure, Electron Transport, Defects and Doping”,    Journal of Display Technology, Vol. 5, No. 7, 2009, pp. 273-288.

SUMMARY OF THE INVENTION

It is known that the conductivity of a semi-conductive oxide varies whenhydrogen or water enters the semi-conductive oxide. Such a phenomenonbecomes a factor of variation in the electrical characteristics of atransistor using the semi-conductive oxide. In addition, electricalcharacteristics of a semiconductor device using the semi-conductiveoxide vary when the semiconductor device is irradiated with visiblelight or ultraviolet light.

In view of the above problems, one object is to provide a semiconductordevice including a semi-conductive oxide film which has stableelectrical characteristics and high reliability. Another object is toprovide a manufacturing process of a semiconductor device, which canmass-produce highly reliable semiconductor devices using a largesubstrate such as mother glass.

It is an object to provide a novel semiconductor device. It is anotherobject to provide a manufacturing method of a novel semiconductordevice.

According to one embodiment of the present invention disclosed in thisspecification, a manufacturing method of a semiconductor device includesthe steps of: forming a first film that contains an oxide including atleast a first metal element and a second metal element, over asubstrate; performing heat treatment on the first film, and forming afirst layer that contains a crystal of an oxide including the firstmetal element as a main metal component and a second layer that is on aside closer to the substrate than the first layer and contains an oxideincluding the second metal element as a main metal component; andforming a second film that is in contact with the first layer andcontains an oxide, and performing heat treatment.

In the above method, the second film may be a semi-conductive oxidefilm. A semi-conductive oxide to be used preferably contains at leastindium (In) or zinc (Zn). In particular, In and Zn are preferablycontained. As a stabilizer for reducing change in electricalcharacteristics of a transistor using the semi-conductive oxide, gallium(Ga) is preferably additionally contained. Tin (Sn) is preferablycontained as a stabilizer. Hafnium (Hf) is preferably contained as astabilizer. Aluminum (Al) is preferably contained as a stabilizer.

As another stabilizer, one or plural kinds of lanthanoid such as lantern(La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm),europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium(Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium (Lu) may becontained.

As the semi-conductive oxide, for example, an indium oxide, a tin oxide,a zinc oxide, a two-component metal oxide such as an In—Zn-based oxide,a Sn—Zn-based oxide, an Al—Zn-based oxide, a Zn—Mg-based oxide, aSn—Mg-based oxide, an In—Mg-based oxide, or an In—Ga-based oxide, athree-component metal oxide such as an In—Ga—Zn-based oxide, anIn—Al—Zn-based oxide, an In—Sn—Zn-based oxide, a Sn—Ga—Zn-based oxide,an Al—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide, an In—Hf—Zn-basedoxide, an In—La—Zn-based oxide, an In—Ce—Zn-based oxide, anIn—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide,an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-basedoxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, anIn—Er—Zn-based oxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide,or an In—Lu—Zn-based oxide, a four-component oxide such as anIn—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, anIn—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, anIn—Sn—Hf—Zn-based oxide, or an In—Hf—Al—Zn-based oxide can be used.

Note that here, for example, an “In—Ga—Zn-based oxide” means an oxidecontaining In, Ga, and Zn as its main metal component, in which there isno particular limitation on the ratio of In:Ga:Zn. In addition to In,Ga, and Zn, a metal element may be contained.

An oxide represented by the chemical formula InMO₃(ZnO)_(m) (m>0) canalso be used. Here, M represents one or more metal elements selectedfrom Zn, Ga, Al, Sn, and In. For example, M can be Ga or two kinds ofmetals such as Ga and Al, Ga and Sn, or Ga and In.

In the case where an In—Zn-based oxide is used as a semi-conductiveoxide, a target to be used has a composition ratio of In:Zn=50:1 to 1:2in an atomic ratio (In₂O₃:ZnO=25:1 to 1:4 in a molar ratio), preferably,In:Zn=1:1 to 1:20 in an atomic ratio (In₂O₃:ZnO=10:1 to 1:2 in a molarratio), further preferably, In:Zn=1.5:1 to 15:1 in an atomic ratio(In₂O₃:ZnO=3:4 to 15:2 in a molar ratio). For example, a target used forthe formation of an In—Zn-based oxide has an atomic ratio ofIn:Zn:O=1:1:X, where X>1, preferably X>1.5.

For example, an In—Ga—Zn-based oxide with an atomic ratio ofIn:Ga:Zn=1:1:1 (=1/3:1/3:1/3) or In:Ga:Zn=2:2:1 (=2/5:2/5:1/5), or anyof oxides whose composition is in the neighborhood of the abovecompositions can be used. Alternatively, an In—Sn—Zn-based oxide with anatomic ratio of In:Sn:Zn=1:1:1 (=1/3:1/3:1/3), In:Sn:Zn=2:1:3(=1/3:1/6:1/2), or In:Sn:Zn=2:1:5 (=1/4:1/8:5/8), or any of oxides whosecomposition is in the neighborhood of the above compositions may beused.

However, the composition is not limited to those described above, and amaterial having the appropriate composition may be used depending onnecessary semiconductor characteristics (e.g., mobility, thresholdvoltage, and variation). In order to obtain necessary semiconductorcharacteristics, it is preferable that the carrier density, the impurityconcentration, the defect density, the atomic ratio of a metal elementto oxygen, the interatomic distance, the density, and the like be set tobe appropriate.

For example, with the In—Sn—Zn-based oxide, a high mobility can berelatively easily obtained. However, the mobility can be increased byreducing the defect density in the bulk also in the case of using theIn—Ga—Zn-based oxide.

Note that for example, the expression “the composition of an oxideincluding In, Ga, and Zn at the atomic ratio, In:Ga:Zn=a:b:c (a+b+c=1),is in the neighborhood of the composition of an oxide including In, Ga,and Zn at the atomic ratio, In:Ga:Zn=A:B:C (A+B+C=1)” means that a, b,and c satisfy the following relation: (a−A)²+(b−B)²+(c−C)²≦r², and r maybe 0.05, for example. The same applies to other oxides.

The semi-conductive oxide may be either single crystal ornon-single-crystal. In the latter case, the semi-conductive oxide may beeither amorphous or polycrystal. Further, the semi-conductive oxide mayhave either an amorphous structure including a portion havingcrystallinity or a non-amorphous structure.

In the above method, the ratio of the second metal element to a metalelement in the second film may be greater than or equal to 0.2. Thefirst film may be formed at a temperature higher than or equal to 200°C. and lower than or equal to 400° C.

In the above method, the first film may be formed by any of thefollowing methods: a microwave plasma sputtering method (a sputteringmethod using a microwave with a frequency of higher than or equal to 100MHz), an RF sputtering method (a sputtering method using anelectromagnetic wave with a frequency of higher than or equal to 1 kHzand lower than 100 MHz), an AC sputtering method (a sputtering methodusing AC with a frequency of lower than 1 kHz, typically, lower than orequal to 100 Hz, the AC sputtering method is also referred to as a cyclesputtering method), and a DC sputtering method (including a sputteringmethod using DC and a pulsed DC sputtering method by which voltage isapplied in a pulsed manner).

In particular, an AC sputtering method or a DC sputtering method ispreferably employed in consideration of mass productivity of formationor the like of a film over a large substrate. In a normal DC sputteringmethod by which constant voltage is applied, abnormal arc discharge isgenerated in some cases. This phenomenon is remarkable in deposition ofa conductive oxide. Voltage is preferably applied in a pulsed manner inorder to prevent the abnormal arc discharge. This method is referred toas a pulsed DC sputtering method. It is not necessary to prevent theabnormal arc discharge in an AC sputtering method; therefore, theproportion of plasma generation time by the AC sputtering method istwice or more as high as that by a pulsed DC sputtering method andfavorable mass productivity is achieved.

In the above method, heat treatment is performed at higher than or equalto 200° C., preferably higher than or equal to 400° C. and lower than700° C. In addition, the atmosphere of the heat treatment may be anatmosphere including oxygen or nitrogen. Further, the pressure may begreater than or equal to 10 Pa and less than or equal to 1 normalatmospheric pressure.

According to one embodiment of the present invention disclosed in thisspecification, a semiconductor device includes: a substrate; aninsulating oxide layer over the substrate; and a semi-conductive oxidelayer provided in contact with the insulating oxide layer. Thesemi-conductive oxide layer includes at least a first metal element anda second metal element. The concentration of the first metal element ina portion in the insulating oxide layer in contact with thesemi-conductive oxide layer is higher than the concentration of thefirst metal element in a portion facing the substrate. The concentrationof the second metal element in the portion in the insulating oxide layerin contact with the semi-conductive oxide layer is lower than theconcentration of the second metal element in the portion facing thesubstrate.

In the above semiconductor device, the first metal element may be zinc.The second metal element may be gallium. In the above semiconductordevice, the semi-conductive oxide layer may include an amorphous state.The semi-conductive oxide layer may include a crystal. Thesemi-conductive oxide layer may be in a single crystal state. Inaddition, the crystal may have a structure in which a c-axis is orientedon a plane perpendicular to the substrate (c-axis-oriented structure).

The present inventor has found a phenomenon in which, when a galliumoxide film containing zinc is subjected to heat treatment, zinc issegregated to a surface to be crystallized. In other words, zinc, whichhas been uniformly distributed in the film initially, is segregated tothe surface by heat treatment, thereby becoming a crystal havingextremely high crystallinity formed mainly from zinc oxide. On the otherhand, the zinc concentration is low enough to obtain sufficientinsulating properties in the other portion.

For example, it can be found from the description of FIG. 12 ofNon-Patent Document 1 that in a substance in which zinc oxide andgallium oxide are mixed with each other in a ratio of 50:50,conductivity of 5×10⁻³ Ω⁻¹ cm⁻¹ is obtained in an amorphous state. Onthe other hand, a substance in which zinc oxide and gallium oxide aremixed with each other in a ratio of 25:75 has a sufficiently insulatingproperty.

This phenomenon is caused because the vapor pressure of zinc is higherthan the vapor pressure of gallium in the above heat treatmentconditions. Therefore, this phenomenon is caused not only when galliumand zinc are used. This phenomenon might be caused if such a conditionis satisfied even when two or more metal elements other than gallium andzinc are combined. For example, an oxide including gallium, aluminum,and zinc may be used instead of an oxide including gallium and zinc.

Further, when the concentration of zinc in the oxide film is measured,the concentration is extremely high in the vicinity of the surface wherea crystal of a zinc oxide is provided, and tends to increase in thedirection to the surface except for the vicinity of the surface. On theother hand, the concentration of gallium tends to decrease in thedirection to the surface. Such a phenomenon is also caused because thevapor pressure of zinc is higher than the vapor pressure of gallium.

In addition, the present inventor has found that, by depositing asemi-conductive oxide which has favorable crystalline consistency withzinc oxide and performing heat treatment at the time when a galliumoxide film is in contact with such a crystal of zinc oxide to become acrystal, a semi-conductive oxide having excellent crystallinity can beobtained at relatively low temperature.

For example, an oxide in which In:Ga:Zn=1:1:N (N is 0.5 or a naturalnumber) has a lattice constant that is very close to that of zinc oxide.When a film having such a composition or a composition close thereto isdeposited over the crystallized zinc oxide and is subjected to heattreatment at 400° C. to 700° C., crystal growth proceeds using zincoxide as a nucleus, whereby a crystal which is highly c-axis-oriented isobtained while reflecting crystallinity of zinc oxide. The crystal maybe highly crystallized to be seen as a single crystal or may include anamorphous state, depending on heating temperature and time.

Such a phenomenon is known as heteroepitaxial growth. In theabove-described example, crystal growth of an In—Ga—Zn-based oxide isperformed using zinc oxide as a nucleus; similarly, crystal growth ofother materials can be conducted.

When a crystalline semi-conductive oxide layer obtained in such a manneris used for a transistor, a transistor which has stable electricalcharacteristics and high reliability can be achieved. Further, when heattreatment temperature is lower than or equal to 450° C., mass productionof highly reliable semiconductor devices using a large glass substratecan be performed.

A process of film formation and a process of heat treatment for forminganother crystalline semi-conductive oxide layer over the crystallinesemi-conductive oxide layer which is obtained in such a manner may berepeated.

With the above-described embodiment, at least one of the problems can beresolved. Even when a transistor having a layered structure of the firstcrystalline semi-conductive oxide layer and the second crystallinesemi-conductive oxide layer is irradiated with light or subjected to abias-temperature stress (BT) test, the amount of change in thresholdvoltage of the transistor can be reduced, and the transistor has stableelectrical characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1F are cross-sectional views of a manufacturing processaccording to Embodiment 1.

FIGS. 2A to 2F are cross-sectional views of a manufacturing processaccording to Embodiment 2.

FIGS. 3A to 3F are cross-sectional views of a manufacturing processaccording to Embodiment 3.

FIGS. 4A to 4F are cross-sectional views of a manufacturing processaccording to Embodiment 4.

FIGS. 5A and 5B are cross-sectional views according to Embodiments 5 and6.

FIGS. 6A to 6C are diagrams illustrating one embodiment of asemiconductor device.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the description below, and it is easilyunderstood by those skilled in the art that modes and details disclosedherein can be modified in various ways without departing from the spiritand the scope of the present invention. Therefore, the present inventionis not construed as being limited to description of the embodiments.

The structures, the conditions, and the like disclosed in any of thefollowing embodiments can be combined with those disclosed in otherembodiments as appropriate. Note that in structures described below, thesame portions or portions having similar functions are denoted by thesame reference numerals in different drawings, and detailed descriptionthereof is not repeated in some cases.

Note that in the following embodiments, an example where the technicalidea of the present invention is applied to a display device including atransistor is mainly given; however, it can be easily understood thatthe technical idea of the present invention is not limited to beingapplied to a display device. Further, terms such as “gate”, “source”,and “drain” used in the embodiments are used for simple description andare not limited to the interpretation of the meanings of the terms.

For example, “a conductive region and a region incorporated therewithprovided so as to get across a semiconductor region over an insulatingfilm provided over the semiconductor region” when normally expressed issimply referred to as a “gate electrode”. Further, a source and a drainare not particularly distinguished in this specification, when one isreferred to as a source, the other is referred to as a drain.

In addition, it should be noted that the terms such as a conductiveoxide, a semi-conductive oxide, and an insulating oxide used in thisspecification do not have absolute meanings. Even when an oxide has thesame composition and the same properties, a name is changed inaccordance with the usage in some cases. For example, when an oxide isused for a target of DC sputtering, it is referred to as a conductiveoxide. When an oxide is used for a semiconductor layer of a transistor,it is referred to as a semi-conductive oxide in some cases.

An oxide in this specification is an oxide in which the percentage ofnitrogen, oxygen, fluorine, sulfur, selenium, chlorine, bromine,tellurium, and iodine (in a molar ratio) contained in a substance(including a compound) is higher than or equal to 25% of the total andthe percentage of oxygen to the above elements (in a molar ratio) ishigher than or equal to 70%.

A metal element in this specification refers to all elements other thana rare gas element, hydrogen, boron, carbon, nitrogen, a Group 16element (e.g., oxygen), a Group 17 element (e.g., fluorine), silicon,phosphorus, germanium, arsenic, and antimony.

Further, in this specification, “one metal element is a main metalelement” indicates the case where among a plurality of metal elements ina substance, the composition of the metal element is greater than orequal to 50% of the metal elements. In addition, “n metal elements M₁,M₂, . . . , and M_(n) are main metal elements” indicates the case wherethe sum of compositions of the metal elements M₁, M₂, . . . , and M_(n)is higher than or equal to {(1-2^(−n))×100}[%] of the metal elements.

Note that the concentration of an element which is not a main componentin a film denoted in this specification is determined by a secondary ionmass spectrometry method unless otherwise specified. In general, whenthe concentration of an element in a depth direction of a single-layeror multilayer film is measured by a secondary ion mass spectrometrymethod, in particular in the case of a small amount of an element, theconcentration of the element tends to be unusually high at an interfacebetween the substrate and the film or between the films; however, theconcentration of such a portion is not an accurate value and measurementvariations are large.

It is desirable that the concentration in the vicinity of an interfacewith low reliability be prevented from being employed and theconcentration of a portion which has a stable concentration be used asan index for the accurate concentration of a film; therefore, as theconcentration determined by a secondary ion mass spectrometry method,the minimum value obtained by analyzing the object is used in thisspecification.

Embodiment 1

In this embodiment, an example in which a display device having atransistor is formed using the above-described technical idea will bedescribed. FIGS. 1A to 1F illustrate a cross section of a manufacturingprocess of the display device of this embodiment. The transistordescribed in this embodiment is a bottom-gate transistor whose gateelectrode is located on the substrate side and a top-contact transistorwhose source electrode and drain electrode are in contact with a topsurface of a semiconductor layer, and a semi-conductive oxide is used asa semiconductor.

The outline of a manufacturing process will be described below. Asillustrated in FIG. 1A, a gate electrode 102, a first gate insulator 103of silicon oxide, silicon oxynitride, or the like, and an oxide film 104are formed over a substrate 101 having an insulating surface. The firstgate insulator 103 is not necessarily provided. In addition, the oxidefilm 104 is an oxide of gallium and zinc in this embodiment, and theratio of gallium, that is, Ga/(Ga+Zn), may be greater than or equal to0.2 and less than 0.8, preferably greater than or equal to 0.3 and lessthan 0.7.

There is no particular limitation on a substrate which can be used forthe substrate 101; however, the substrate needs to have an insulatingsurface. For example, a glass substrate made of barium borosilicateglass, aluminoborosilicate glass, or the like can be used; however, oneembodiment of the present invention is not limited to this. An insulatorsuch as quartz or sapphire, or a semiconductor having sufficiently highinsulating properties, such as silicon carbide, may be used. Further, aninsulating film may be formed on a surface of a semiconductor whoseinsulating properties are not high such as silicon, germanium, orgallium arsenide, on a surface of a semiconductor whose conductivity isincreased by doping, or on a surface of copper, aluminum, or the like.

In the case where unfavorable impurities for a transistor are includedin a substrate, it is preferable that a film of an insulating materialhaving a function of blocking the impurities (e.g., aluminum nitride,aluminum oxide, or silicon nitride) be provided on a surface. Note thatin this embodiment, the first gate insulator 103 can have a similarfunction.

The gate electrode 102 can be formed in a single layer or a stackedlayer using a metal element such as molybdenum, titanium, chromium,tantalum, tungsten, aluminum, or copper, or an alloy material whichincludes any of these materials as a main metal element. Because thethreshold value or the like of the obtained transistor is changed due toa work function of a material to be used for the gate electrode 102, itis necessary to select a material in accordance with the thresholdvoltage which is required.

It is necessary to determine the thickness of the first gate insulator103 in accordance with the composition and thickness of the oxide film104. Descriptions thereof will be made later. The first gate insulator103 may be formed by a known sputtering method, a CVD method, or thelike.

The oxide film 104 is formed by a microwave plasma sputtering method, anRF plasma sputtering method, an AC sputtering method, or a DC sputteringmethod. Determination of which method is to be employed may be performedin consideration of the conductivity of a target, the size of thetarget, the area of a substrate, or the like.

A target to be used may be an oxide in which the ratio of gallium tozinc is adjusted so that the oxide film 104 takes the above-mentionedvalue. Note that in sputtering, the composition of the target isdifferent from the composition of the obtained film depending on anatmosphere and temperature of a deposition surface; for example, evenwhen a conductive target is used, the concentration of zinc of theobtained film is decreased, so that the obtained film has insulatingproperties or semiconductivity in some cases.

In this embodiment, an oxide of zinc and gallium is used; the vaporpressure of zinc at higher than or equal to 200° C. is higher than thatof gallium. Therefore, when the substrate 101 is heated at higher thanor equal to 200° C., the concentration of zinc of the oxide film 104 islower than the concentration of zinc of the target. Accordingly, inconsideration of the fact, it is necessary that the concentration ofzinc of the target be set at a higher concentration. When theconcentration of zinc is increased in general, the conductivity of anoxide is improved; therefore, a DC sputtering method is preferably used.

The target for sputtering can be obtained in such a manner that after apowder of gallium oxide and a powder of zinc oxide are mixed andpre-baked, molding is performed; then, baking is performed.Alternatively, it is preferable that a powder of gallium oxide whosegrain size is less than or equal to 100 nm and a powder of zinc oxidewhose grain size is less than or equal to 100 nm be sufficiently mixedand molded.

The oxide film 104 is desirably formed by a method in which hydrogen,water, or the like does not easily enter the oxide film 104. Theatmosphere in film formation may be a rare gas (typically argon)atmosphere, an oxygen atmosphere, a mixed atmosphere of a rare gas andoxygen, or the like. Moreover, it is desirably an atmosphere using ahigh-purity gas from which impurities such as hydrogen, water, ahydroxyl group, and hydride are sufficiently removed because entry ofhydrogen, water, a hydroxyl group, and hydride into the oxide film 104can be prevented.

The entry of the impurities can also be prevented when the substratetemperature in film formation is set to higher than or equal to 100° C.and lower than or equal to 600° C., preferably higher than or equal to200° C. and lower than or equal to 400° C. In addition, an entrapmentvacuum pump such as a cryopump, an ion pump, or a titanium sublimationpump or a turbo molecular pump provided with a cold trap may be used asan evacuation unit.

In the deposition chamber which is evacuated with the above-describedevacuation unit, a hydrogen molecule, a compound containing a hydrogenatom such as water (H₂O), (preferably, also a compound containing acarbon atom), and the like are removed. Accordingly, the concentrationof impurities in the oxide film 104 formed in the deposition chamber canbe reduced.

Next, the substrate 101 provided with these is heated at 400° C. to 700°C. for 10 minutes to 24 hours under an appropriate atmosphere, forexample, under the condition that the pressure is 10 Pa to 1 normalatmospheric pressure and an atmosphere is any of an oxygen atmosphere, anitrogen atmosphere, and a mixed atmosphere of oxygen and nitrogen.Then, the quality of the oxide film 104 is changed as illustrated inFIG. 1B, and a semi-conductive oxide layer 104 a having highconcentration of zinc is formed in the vicinity of a surface of theoxide film 104, and another portion becomes an insulating oxide layer104 b having low concentration of zinc.

Note that as a heating period is longer, heating temperature is higher,and the pressure in heating is lower, zinc is easily evaporated and thesemi-conductive oxide layer 104 a tends to be thin.

The thickness of the semi-conductive oxide layer 104 a is preferably 3nm to 15 nm. The thickness of the semi-conductive oxide layer 104 a canbe controlled by heating time, heating temperature, and pressure at thetime of heating as described above, or by the composition and thicknessof the oxide film 104. The composition of the oxide film 104 can becontrolled by substrate temperature in film formation, as well as thecomposition of the target; therefore, these may be set as appropriate.

The obtained semi-conductive oxide layer 104 a has crystallinity; in anX-ray diffraction analysis of a crystal structure, the ratio of thediffraction intensity of an a plane or a b plane to the diffractionintensity of a c plane is greater than or equal to 0 and less than orequal to 0.3, showing c-axis orientation properties. In this embodiment,the semi-conductive oxide layer 104 a is an oxide in which zinc is amain metal component.

On the other hand, the ratio of gallium, that is, Ga/(Ga+Zn) in theinsulating oxide layer 104 b may be greater than or equal to 0.7,preferably greater than or equal to 0.8. Note that the ratio of galliumin the insulating oxide layer 104 b in a portion close to the surface,for example, in a portion in contact with the semi-conductive oxidelayer 104 a has the lowest value and the ratio is increased toward thesubstrate. In contrast, the ratio of zinc in the portion close to thesurface has the highest value and the ratio is decreased toward thesubstrate.

Note that in this heat treatment, an alkali metal such as lithium,sodium, or potassium is also segregated in the vicinity of the surfaceof the semi-conductive oxide layer 104 a and evaporated; therefore, theconcentration in the semi-conductive oxide layer 104 a and theconcentration in the insulating oxide layer 104 b are sufficientlyreduced. These alkali metals are unfavorable elements for a transistor;thus, it is preferable that these alkali metals be contained in amaterial used for forming the transistor as little as possible. Sincethese alkali metals are easily evaporated compared to zinc; therefore, aheat treatment step is advantageous in removing these alkali metals.

By such treatment, for example, the concentration of sodium in each ofthe semi-conductive oxide layer 104 a and the insulating oxide layer 104b may be less than or equal to 5×10¹⁶ cm⁻³, preferably less than orequal to 1×10¹⁶ cm⁻³, more preferably less than or equal to 1×10¹⁵ cm⁻³.Similarly, the concentration of lithium in each of the semi-conductiveoxide layer 104 a and the insulating oxide layer 104 b may be less thanor equal to 5×10¹⁵ cm⁻³, preferably less than or equal to 1×10¹⁵ cm⁻³;the concentration of potassium in each of the semi-conductive oxidelayer 104 a and the insulating oxide layer 104 b may be less than orequal to 5×10¹⁵ cm⁻³, preferably less than or equal to 1×10¹⁵ cm⁻³.

The insulating oxide layer 104 b obtained in this manner functions as agate insulator of the transistor. In other words, the thickness of thegate insulator of the transistor is the sum of the thickness of thefirst gate insulator 103 and the thickness of the insulating oxide layer104 b. Therefore, the thickness of the first gate insulator 103 needs tobe determined in consideration of the insulating oxide layer 104 b.

The thickness of the insulating oxide layer 104 b depends not only onthe thickness of the oxide film 104 but also on the ratio of zinccontained in the oxide film 104. In general, as the ratio of zinc ishigher, the insulating oxide layer 104 b becomes thinner. Therefore, thethickness of the first gate insulator 103 needs to be determined inaccordance with the composition and thickness of the oxide film 104, asdescribed above.

For example, in the case of a transistor used for a general liquidcrystal display device or a general electroluminescence display device,the thickness of a gate insulator is 50 nm to 1 μm. For example, in thecase where the thickness of the oxide film 104 is 200 nm and the ratioof gallium, that is, Ga/(Ga+Zn) in the oxide film 104 is 0.5, thethickness of the obtained insulating oxide layer 104 b is 100 nm to 150nm. Note that the dielectric constant of the insulating oxide layer 104b is approximately 2.5 times as high as that of silicon oxide becausegallium is a main metal element.

In the case where the first gate insulator 103 is formed using siliconoxide and the silicon oxide equivalent thickness of the total gateinsulator (the first gate insulator 103 and the insulating oxide layer104 b) is 200 nm, the thickness of the first gate insulator 103 may be140 nm to 160 nm.

Note that the optimum thickness of the gate insulator is set by voltageapplied to the gate electrode or the like, as appropriate. In general,in the case where the applied voltage is low, the gate insulator is setto be thin, whereas in the case where the applied voltage is high, thegate insulator is set to be thick.

In this embodiment, the insulating oxide layer 104 b in which gallium isa main metal element is represented by a chemical formula:Ga_(x)Zn_(1-x)O_(y) (note that X≧0.7); however, it is preferable thatoxygen exceed a stoichiometric ratio so as to satisfy x/2+1<y<x/2+1.5.

Note that an impurity element, e.g., a Group 3 element such as yttrium,a Group 4 element such as hafnium, or a Group 13 element such asaluminum is contained in the oxide film 104, the energy gap of theinsulating oxide layer 104 b to be obtained later may be increased andthe insulating properties may be enhanced. The energy gap of galliumoxide which does not contain any of the above impurities is 4.9 eV;however, when the gallium oxide contains any of the above impurities atabout, for example, greater than 0 atomic % and less than or equal to 20atomic %, the energy gap can be increased to about 6 eV.

Then, a semi-conductive oxide film 105 is formed as illustrated in FIG.1C. In this embodiment, an In—Ga—Zn-based oxide is used as asemi-conductive oxide. In other words, the semi-conductive oxide film isformed using an In—Ga—Zn-based oxide as a target by a sputtering method.The filling rate of the oxide target is higher than or equal to 90% andlower than or equal to 100%, preferably, higher than or equal to 95% andlower than or equal to 99.9%. With the use of the oxide target with highfilling rate, a semi-conductive oxide film to be obtained can have highdensity.

The composition ratio of the target can be, for example, In:Ga:Zn=1:1:1[molar ratio]. Note that it is not necessary to limit the material andcomposition ratio of the target to the above. For example, an oxidetarget with the following composition ratio may alternatively be used:In:Ga:Zn=1:1:0.5 [molar ratio], In:Ga:Zn=2:1:3 [molar ratio], andIn:Ga:Zn=3:1:2 [molar ratio].

As described later, as for the composition of an obtainedsemi-conductive oxide film, it is preferable that the ratio of galliumin a metal component (molar ratio) be greater than or equal to 0.2. Forexample, in the case where In:Ga:Zn=1:1:1, the ratio of gallium is 0.33,whereas in the case where In:Ga:Zn=1:1:0.5, the ratio of gallium is 0.4.

The semi-conductive oxide film 105 is desirably formed by a method inwhich hydrogen, water, or the like does not easily enter thesemi-conductive oxide film 105. The atmosphere in film formation may bea rare gas (typically argon) atmosphere, an oxygen atmosphere, a mixedatmosphere of a rare gas and oxygen, or the like. An atmosphere of ahigh-purity gas from which an impurity such as hydrogen, water, ahydroxyl group, or hydride is removed is preferable, in order to preventhydrogen, water, a hydroxyl group, hydride, or the like from enteringthe semi-conductive oxide film 105.

The thickness of the semi-conductive oxide film 105 is desirably greaterthan or equal to 3 nm and less than or equal to 30 nm. This is becausewhen the thickness of the semi-conductive oxide film is too large (e.g.,when the thickness is greater than or equal to 50 nm), the transistormight be normally on.

The entry of the impurities can also be prevented when the substratetemperature in film formation is set to higher than or equal to 100° C.and lower than or equal to 600° C., preferably higher than or equal to200° C. and lower than or equal to 400° C. In addition, an entrapmentvacuum pump such as a cryopump, an ion pump, or a titanium sublimationpump or a turbo molecular pump provided with a cold trap may be used asan evacuation unit.

In the deposition chamber which is evacuated with the above-describedevacuation unit, a hydrogen molecule, and a compound containing ahydrogen atom such as water (H₂O) as well as a compound containing acarbon atom are removed. Accordingly, the concentration of impurities inthe semi-conductive oxide film 105 formed in the deposition chamber canbe reduced.

An alkali metal such as lithium, sodium, or potassium or analkaline-earth metal is unfavorable element for the case where asemi-conductive oxide is used for a transistor; therefore, it ispreferable that an alkali metal or an alkaline-earth metal be containedin a material used for forming the transistor as little as possible.

Of alkali metals, in particular, sodium is dispersed in an insulatingoxide which is in contact with a semi-conductive oxide to be a sodiumion. Alternatively, sodium cuts a bond between a metal element andoxygen or enters the bond in the semi-conductive oxide. As a result,transistor characteristics deteriorate (e.g., the transistor becomesnormally-on (the shift of a threshold voltage to a negative side) or themobility is decreased). In addition, this also causes variation in thecharacteristics.

Such a problem is significant especially in the case where theconcentration of hydrogen in the semi-conductive oxide is extremely low.Therefore, the concentration of an alkali metal is strongly required tobe extremely low in the case where the concentration of hydrogen in thesemi-conductive oxide is lower than or equal to 5×10¹⁹ cm⁻³,particularly lower than or equal to 5×10¹⁸ cm⁻³.

For example, the concentration of sodium in the semi-conductive oxidefilm 105 may be less than or equal to 5×10¹⁶ cm⁻³, preferably less thanor equal to 1×10¹⁶ cm⁻³, more preferably less than or equal to 1×10¹⁵cm⁻³. Similarly, the concentration of lithium in the semi-conductiveoxide film 105 may be less than or equal to 5×10¹⁵ cm⁻³, preferably lessthan or equal to 1×10¹⁵ cm⁻³; the concentration of potassium in thesemi-conductive oxide film 105 may be less than or equal to 5×10¹⁵ cm⁻³,preferably less than or equal to 1×10¹⁵ cm⁻³.

Then, heat treatment (first heat treatment) is performed on thesemi-conductive oxide film 105. By the first heat treatment, crystalgrowth of the semi-conductive oxide film 105 is carried out using acrystal of the semi-conductive oxide layer 104 a as a nucleus, and asemi-conductive oxide film 105 a which is c-axis-oriented is obtained asin FIG. 1D.

At the same time, excessive hydrogen (including water and a hydroxylgroup) in the semi-conductive oxide film 105 can be removed; thestructure of the semi-conductive oxide film 105 can be improved; anddefect levels in the energy gap can be reduced.

Further, excessive hydrogen (including water and a hydroxyl group) inthe first gate insulator 103 and the insulating oxide layer 104 b canalso be removed by the first heat treatment. The temperature of thefirst heat treatment is higher than or equal to 250° C. and lower thanor equal to 650° C.

As a result of the first heat treatment, the semi-conductive oxide layer104 a and the semi-conductive oxide film 105 are integrated with eachother to be the semi-conductive oxide film 105 a; therefore, aninterface between the semi-conductive oxide layer 104 a and thesemi-conductive oxide film 105 is not clear.

The first heat treatment can be performed in such a manner that, forexample, an object is introduced into an electric furnace in which aresistance heating element or the like is used and heated under anitrogen atmosphere. During the first heat treatment, thesemi-conductive oxide film 105 is not exposed to air to prevent entry ofwater and hydrogen.

The heat treatment apparatus is not limited to the electric furnace andmay be an apparatus for heating an object by thermal radiation orthermal conduction from a medium such as a heated gas. For example, arapid thermal anneal (RTA) apparatus such as a gas rapid thermal anneal(GRTA) apparatus or a lamp rapid thermal anneal (LRTA) apparatus can beused.

An LRTA apparatus is an apparatus for heating an object by radiation oflight (an electromagnetic wave) emitted from a lamp such as a halogenlamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, a highpressure sodium lamp, or a high pressure mercury lamp. A GRTA apparatusis an apparatus for performing heat treatment using a high-temperaturegas. As the gas, an inert gas which does not react with an object byheat treatment, such as nitrogen or a rare gas such as argon is used.

For example, as the first heat treatment, the GRTA process may beperformed as follows. The object is put in a heated inert gasatmosphere, heated for several minutes, and taken out of the inert gasatmosphere. The GRTA process enables high-temperature heat treatment fora short time. Moreover, the GRTA process can be employed even when thetemperature exceeds the upper temperature limit of the object. Note thatthe inert gas may be switched to a gas including oxygen during theprocess. This is because defect levels in the energy gap due to oxygendeficiency can be reduced by performing the first heat treatment in anatmosphere including oxygen.

Note that as the inert gas atmosphere, an atmosphere that containsnitrogen or a rare gas (e.g., helium, neon, or argon) as its maincomponent and does not contain water, hydrogen, or the like ispreferably used. For example, the purity of nitrogen or a rare gas suchas helium, neon, or argon introduced into a heat treatment apparatus isgreater than or equal to 6 N (99.9999%), preferably greater than orequal to 7 N (99.99999%) (that is, the concentration of the impuritiesis less than or equal to 1 ppm, preferably less than or equal to 0.1ppm).

The first heat treatment can be implemented not only just after thesemi-conductive oxide film 105 is formed as described above but also atany timing after the semi-conductive oxide film 105 is formed. Inaddition, similar heat treatment may be performed plural times insteadof one.

As illustrated in FIG. 1E, the semi-conductive oxide film 105 a and theinsulating oxide layer 104 b are etched, whereby a semi-conductive oxidefilm 105 b having a desired shape (e.g., island shape) is obtained. Inthe etching, a dry etching method or a wet etching method may be used.Note that as the etching here, the first gate insulator 103 may be usedas an etching stopper.

Then, a semi-conductive oxide film having N-type conductivity and aconductive film of a metal or the like are deposited. For the formationof these films, a sputtering method may be used. For the N-typesemi-conductive oxide film, indium oxide, indium tin oxide, zinc oxide,zinc aluminum oxide, or the like may be used. Note that the N-typesemi-conductive oxide film is provided for reducing contact resistancebetween a source electrode and a drain electrode and the semi-conductiveoxide film 105 b; however, the N-type semi-conductive oxide film is notnecessarily provided depending on the kinds of metals to be used for thesource electrode and the drain electrode.

As the conductive film, for example, a metal film containing an elementselected from aluminum, chromium, copper, tantalum, titanium,molybdenum, or tungsten, or a metal nitride film containing any of theabove elements as a main metal component (e.g., a titanium nitride film,a molybdenum nitride film, or a tungsten nitride film) can be used.

Alternatively, a film of a high-melting-point metal such as titanium,molybdenum, or tungsten or a metal nitride film (e.g., a titaniumnitride film, a molybdenum nitride film, or a tungsten nitride film) maybe formed over or/and below the metal film such as an Al film or a Cufilm.

Then, these are processed into desired shapes, and N-typesemi-conductive oxide films 106 a and 106 b, a source electrode 107 a,and a drain electrode 107 b are formed. In the above manner, a basicstructure of a transistor is completed. Note that in etching of theconductive film, part of the semi-conductive oxide film 105 b is etchedand a groove portion (depression portion) is formed in thesemi-conductive oxide film 105 b, in some cases.

Plasma treatment may be performed using an oxidizing gas such as oxygenor ozone, and the adsorbed water attached to the surface of the exposedsemi-conductive oxide film 105 b may be removed. Note that in the plasmatreatment, it is preferable that the concentration of nitrogen or thatof argon in the gas be less than 50%.

Further, a first insulator 108 is formed by a sputtering method, a CVDmethod, or the like. In the case where the plasma treatment isperformed, it is preferable that the first insulator 108 be successivelyformed without the substrate 101 being taken out to an air atmosphereafter the plasma treatment because an atmospheric component (inparticular, water) is not adsorbed on the surface of the semi-conductiveoxide film 105 b.

The first insulator 108 can be formed typically using an inorganicinsulator such as silicon oxide, silicon oxynitride, aluminum oxide, oraluminum oxynitride. In particular, it is preferable to use an oxidebecause of the reason to be described below, and oxygen that isequivalent to or exceeds the stoichiometric ratio is preferablycontained.

The second heat treatment is preferably performed after the firstinsulator 108 is formed. The second heat treatment is performed at atemperature higher than or equal to 150° C. and lower than or equal to600° C., preferably higher than or equal to 250° C. and lower than orequal to 450° C.

The second heat treatment may be performed in an atmosphere of nitrogen,oxygen, ultra-dry air (air in which the water content is less than orequal to 20 ppm, preferably less than or equal to 1 ppm, more preferablyless than or equal to 10 ppb), or a rare gas (argon, helium, or thelike). It is also preferable that the purity of nitrogen, oxygen, or therare gas which is introduced into a heat treatment apparatus be set tobe greater than or equal to 6 N, preferably greater than or equal to 7N(that is, the impurity concentration is less than or equal to 1 ppm,preferably less than or equal to 0.1 ppm).

In the second heat treatment, the semi-conductive oxide film 105 b andthe first insulator 108 are heated in a state where the semi-conductiveoxide film 105 b is in contact with the first insulator 108. Therefore,oxygen in the semi-conductive oxide film 105 b, which might be reducedby the first heat treatment, can be supplied from the first insulator108. Accordingly, charge trapping centers in the semi-conductive oxidefilm 105 b can be decreased.

The first heat treatment and the second heat treatment are applied,whereby the semi-conductive oxide film 105 b can be highly purified soas to contain impurities other than main components as little aspossible. The highly purified semi-conductive oxide film 105 b containsextremely few carriers derived from a donor. The carrier concentrationcan be less than 1×10¹⁴/cm³, preferably less than 1×10¹²/cm³, and morepreferably less than 1×10¹¹/cm³.

Next, a second insulator 109 which has a flat surface is formed. Variousorganic materials may be used for forming the second insulator 109.Then, the first insulator 108 and the second insulator 109 areselectively etched, so that a contact hole reaching the drain electrode107 b is formed. A display electrode 110 which is in contact with thedrain electrode 107 b through this contact hole is formed (FIG. 1F).

A light-transmitting material or a reflective material can be used forthe display electrode 110. For the light-transmitting material, aconductive oxide whose band gap is greater than or equal to 3electron-volts, such as an In—Sn-based oxide or a Zn—Al-based oxide, canbe used. A metal nanowire or a carbon film (graphene or the like) with athickness of less than or equal to 3 nm can also be used. For thereflective material, a film formed using any of various metal materials(aluminum, silver, and the like) can be used. The surface of areflective display electrode is preferably provided with an irregularunevenness to display white color.

FIGS. 6A to 6C illustrate states of the above-described manufacturingprocess which are seen from above. A cross section taken along a dashedline connecting a point A and a point B in each of FIGS. 6A to 6Ccorresponds to FIGS. 1A to 1F. FIG. 6A corresponds to the stateillustrated in FIG. 1A; here, the first gate insulator 103, the oxidefilm 104, and the like are not illustrated. FIG. 6B corresponds to thestate illustrated in FIG. 1E. In addition, FIG. 6C illustrates anintermediate state between the step illustrated in FIG. 1E and the stepillustrated in FIG. 1F. In other words, FIG. 6C illustrates a state justafter the N-type semi-conductive oxide films 106 a and 106 b, the sourceelectrode 107 a, and the drain electrode 107 b are formed after the stepillustrated in FIG. 1E.

In this embodiment, the insulating oxide layer 104 b in which gallium isa main metal element is used. When such a material is in contact with asemi-conductive oxide in which, in particular, the ratio of gallium in ametal element is greater than or equal to 0.2, charge trapping at aninterface between the insulating oxide layer 104 b and a semi-conductiveoxide film can be sufficiently suppressed. Accordingly, a highlyreliable semiconductor device can be provided.

In this embodiment, the manufacturing process of the display deviceusing a transistor is described; it is apparent that the methoddisclosed in this embodiment can be applied not only to a display devicebut also an electronic device of another embodiment (e.g., an integratedcircuit).

Embodiment 2

In this embodiment, an example in which a display device including atransistor having a different structure from the transistor described inEmbodiment 1 is manufactured will be described. FIGS. 2A to 2F arecross-sectional views illustrating a manufacturing process of thedisplay device of this embodiment. The transistor described in thisembodiment includes a semi-conductive oxide as a semiconductor and is abottom-gate transistor. In addition, the transistor described in thisembodiment is a bottom-contact transistor whose source electrode anddrain electrode are in contact with a lower surface of a semiconductorlayer.

The outline of a manufacturing process will be described below. Notethat for the structures denoted by the same reference numerals as thosein Embodiment 1, a material, a means, a condition, and the likedescribed in Embodiment 1 may be used as those in this embodiment unlessotherwise specified. As illustrated in FIG. 2A, the gate electrode 102,the first gate insulator 103, and the oxide film 104 are formed over thesubstrate 101 having an insulating surface.

The thickness of the first gate insulator 103 needs to be determined inaccordance with the composition and thickness of the oxide film 104 forthe same reason described in Embodiment 1. The first gate insulator 103is not necessarily provided. The oxide film 104 is an oxide of galliumand zinc in this embodiment, and the ratio of gallium, that is,Ga/(Ga+Zn), may be greater than or equal to 0.2 and less than 0.8,preferably greater than or equal to 0.3 and less than 0.7.

Next, the substrate 101 provided with these is heated at 400° C. to 700°C. for 10 minutes to 24 hours under an appropriate atmosphere, forexample, under the condition that the pressure is 10 Pa to 1 normalatmospheric pressure and an atmosphere is any of an oxygen atmosphere, anitrogen atmosphere, and a mixed atmosphere of oxygen and nitrogen.Then, the quality of the oxide film 104 is changed as illustrated inFIG. 2B, and the crystalline semi-conductive oxide layer 104 a havinghigh concentration of zinc is formed in the vicinity of a surface of theoxide film 104, and another portion becomes the insulating oxide layer104 b having low concentration of zinc. The ratio of gallium, that is,Ga/(Ga+Zn), may be greater than or equal to 0.7, preferably greater thanor equal to 0.8 in the insulating oxide layer 104 b.

After that, a conductive film such as a metal film is deposited andprocessed to have a desired shape, so that the source electrode 107 aand the drain electrode 107 b are formed as illustrated in FIG. 2C.Further, the semi-conductive oxide film is formed thereover, and thesemi-conductive oxide film and the insulating oxide layer 104 b areetched, whereby the semi-conductive oxide film 105 b having a desiredshape (e.g., island shape) is obtained as illustrated in FIG. 2D. In theetching, a dry etching method or a wet etching method may be used.

Further, the first heat treatment described in Embodiment 1 isperformed, and in particular, the semi-conductive oxide film 105 b in aportion in contact with the semi-conductive oxide layer 104 a is made tobe crystallized, whereby the semi-conductive oxide film 105 a isobtained (see FIG. 2E).

Further, the first insulator 108 is formed. After the first insulator108 is formed, the second heat treatment may be performed. Next, thesecond insulator 109 which has a flat surface is formed. Then, the firstinsulator 108 and the second insulator 109 are selectively etched, sothat a contact hole reaching the drain electrode 107 b is formed. Thedisplay electrode 110 which is in contact with the drain electrode 107 bthrough this contact hole is formed (FIG. 2F).

The difference between the transistor described in this embodiment andthat described in Embodiment 1 is only positional relation between thesemi-conductive oxide film 105, and the source electrode 107 a and thedrain electrode 107 b. Therefore, the structure of the transistor seenfrom the above is almost the same as the structure illustrated in FIG.6C.

Also in this embodiment, the insulating oxide layer 104 b in whichgallium is a main metal element is used. When such a material is incontact with a semi-conductive oxide in which, in particular, the ratioof gallium in a metal element is greater than or equal to 0.2, chargetrapping at an interface between the insulating oxide layer 104 b and asemi-conductive oxide film can be sufficiently suppressed. Accordingly,a highly reliable semiconductor device can be provided.

In this embodiment, the manufacturing process of the display deviceusing a transistor is described; it is apparent that the methoddisclosed in this embodiment can be applied not only to a display devicebut also an electronic device of another embodiment (e.g., an integratedcircuit).

Embodiment 3

In this embodiment, an example in which a display device including atransistor having a different structure from the transistors describedin the above embodiments is manufactured will be described. FIGS. 3A to3F are cross-sectional views illustrating a manufacturing process of thedisplay device of this embodiment. The transistor described in thisembodiment includes a semi-conductive oxide as a semiconductor and is atop-gate transistor in which a gate is formed over a semiconductorlayer. In addition, the transistor described in this embodiment is atop-contact transistor whose source electrode and drain electrode are incontact with an upper surface of a semiconductor layer.

The outline of a manufacturing process will be described below. Notethat for the structures denoted by the same reference numerals as thosein Embodiment 1, a material, a means, a condition, and the likedescribed in Embodiment 1 may be used as those in this embodiment unlessotherwise specified. As illustrated in FIG. 3A, an oxide film 111 isformed over the substrate 101. Note that since the oxide film 111becomes an insulating oxide through a step such as heat treatmentperformed later, the surface of the substrate 101 may have conductivity.

Further, in the case where unfavorable impurities for a transistor areincluded in a substrate, it is preferable that a film of an insulatingmaterial having a function of blocking the impurities (e.g., aluminumnitride, aluminum oxide, or silicon nitride) be provided between thesubstrate 101 and the oxide film 111. Note that depending on the kind ofthe oxide film 111, the oxide film 111 can have a similar function inheat treatment performed later.

The oxide film 111 is an oxide of gallium and zinc in this embodiment,and the ratio of gallium, that is, Ga/(Ga+Zn), may be greater than orequal to 0.2 and less than 0.8, preferably greater than or equal to 0.3and less than 0.7. The oxide film 111 is formed by a DC sputteringmethod or a pulsed DC sputtering method. The oxide film 111 can beformed in a manner similar to that of the oxide film 104 in Embodiment1.

Next, the substrate 101 is heated at 400° C. to 700° C. for 10 minutesto 24 hours under an appropriate atmosphere, for example, under thecondition that the pressure is 10 Pa to 1 normal atmospheric pressureand an atmosphere is any of an oxygen atmosphere, a nitrogen atmosphere,and a mixed atmosphere of oxygen and nitrogen. Then, the quality of theoxide film 111 is changed as illustrated in FIG. 3B, and a crystallinesemi-conductive oxide layer 111 a having high concentration of zinc isformed in the vicinity of a surface of the oxide film 111, and anotherportion becomes an insulating oxide layer 111 b having low concentrationof zinc. The ratio of gallium, that is, Ga/(Ga+Zn), may be greater thanor equal to 0.7, preferably greater than or equal to 0.8 in theinsulating oxide layer 111 b.

In this embodiment, the insulating oxide layer 111 b in which gallium isa main metal element is represented by the chemical formula:Ga_(x)Zn_(1-x)O_(y) (note that X÷0.7); however, it is preferable thatoxygen exceed the stoichiometric ratio so as to satisfy x/2+1<y<x/2+1.5.

In this embodiment, since the insulating oxide layer 111 b is an oxidein which gallium is a main metal element, the insulating oxide layer 111b has a function of blocking hydrogen and an alkali metal.

After that, the semi-conductive oxide film 105 is formed over thesemi-conductive oxide layer 111 a as illustrated in FIG. 3C. For theformation condition and the like of the semi-conductive oxide film 105,refer to Embodiment 1. Further, the semi-conductive oxide film 105 iscrystallized by performing the first heat treatment described inEmbodiment 1, whereby the semi-conductive oxide film 105 a is obtained(see FIG. 3D).

Then, the semi-conductive oxide film 105 is etched, whereby thesemi-conductive oxide film 105 b having a desired shape (e.g., islandshape) is obtained. In the etching, a dry etching method or a wetetching method may be used. Note that as the etching here, theinsulating oxide layer 111 b may be used as an etching stopper. Afterthat, the plasma treatment described in Embodiment 1 may be performed.

After that, a semi-conductive oxide film having n-type conductivity anda conductive film such as a metal film are deposited and each processedto have a desired shape, so that the N-type semi-conductive oxide films106 a and 106 b, the source electrode 107 a, and the drain electrode 107b are formed (see FIG. 3E). Note that the N-type semi-conductive oxidefilms 106 a and 106 b are not necessarily provided.

Further, a gate insulator 112 is deposited. For a formation method ofthe gate insulator 112, refer to the formation method of the first gateinsulator 103 in Embodiment 1. The thickness of the gate insulator 112may be set to a thickness which a transistor to be formed needs.

A gate electrode 113 is formed over the gate insulator 112. The gateelectrode 113 can be a single layer or a stacked layer using a metalelement such as molybdenum, titanium, chromium, tantalum, tungsten,aluminum, or copper, or an alloy material which includes any of theseelements as a main component. Because the threshold value or the like ofthe obtained transistor is changed by a work function of a material usedfor the gate electrode 113, selection of a material in accordance withthe required threshold is needed. In the above manner, a basic structureof a transistor is completed.

Further, the first insulator 108 is formed by a sputtering method, a CVDmethod, or the like. After the first insulator 108 is formed, the secondheat treatment may be performed. Next, the second insulator 109 whichhas a flat surface is formed. Then, the first insulator 108 and thesecond insulator 109 are selectively etched, so that a contact holereaching the drain electrode 107 b is formed. The display electrode 110which is in contact with the drain electrode 107 b through this contacthole is formed (see FIG. 3F).

The large difference between the transistor described in this embodimentand that described in Embodiment 1 is positional relation between thesemi-conductive oxide film 105 and the gate electrode 113. However, theposition of the gate electrode 102 in a substrate surface in FIG. 1F isalmost the same as that of the gate electrode 113 in FIG. 3F. Therefore,the structure of the transistor seen from the above is almost the sameas the structure illustrated in FIG. 6C.

Also in this embodiment, the insulating oxide layer 111 b in whichgallium is a main metal element is used. When such a material is incontact with a semi-conductive oxide film in which, in particular, theratio of gallium in a metal element is greater than or equal to 0.2,charge trapping at an interface between the insulating oxide layer 111 band the semi-conductive oxide film can be sufficiently suppressed.Accordingly, a highly reliable semiconductor device can be provided.

In this embodiment, the manufacturing process of the display deviceusing a transistor is described; it is apparent that the methoddisclosed in this embodiment can be applied not only to a display devicebut also an electronic device of another embodiment (e.g., an integratedcircuit).

Embodiment 4

In this embodiment, an example in which a display device including atop-gate transistor like the transistor described in Embodiment 3 ismanufactured is described; however the transistor in this embodiment isa bottom-contact transistor whose source electrode and drain electrodeare in contact with a lower surface of a semiconductor layer. FIGS. 4Ato 4F are cross-sectional views illustrating a manufacturing process ofthe display device of this embodiment.

The outline of a manufacturing process will be described below. Notethat for the structures denoted by the same reference numerals as thosein Embodiment 1 or 3, those described in Embodiment 1 or 3 may be usedunless otherwise specified. As illustrated in FIG. 4A, the oxide film111 is formed over the substrate 101. The surface of the substrate 101may have conductivity as in Embodiment 3.

The oxide film 111 is an oxide of gallium and zinc in this embodiment,and the ratio of gallium, that is, Ga/(Ga+Zn), may be greater than orequal to 0.2 and less than 0.8, preferably greater than or equal to 0.3and less than 0.7. The thickness of the oxide film 111 may be more thanor equal to 100 nm and less than or equal to 1000 nm.

Next, the substrate 101 is heated at 400° C. to 700° C. for 10 minutesto 24 hours under an appropriate atmosphere, for example, under thecondition that the pressure is 10 Pa to 1 normal atmospheric pressureand an atmosphere is any of an oxygen atmosphere, a nitrogen atmosphere,and a mixed atmosphere of oxygen and nitrogen. Then, the quality of theoxide film 111 is changed as illustrated in FIG. 4B, and the crystallinesemi-conductive oxide layer 111 a having high concentration of zinc isformed in the vicinity of a surface of the oxide film 111, and anotherportion becomes the insulating oxide layer 111 b having lowconcentration of zinc. The ratio of gallium, that is, Ga/(Ga+Zn), may begreater than or equal to 0.7, preferably greater than or equal to 0.8 inthe insulating oxide layer 111 b.

After that, a conductive film such as a metal film is deposited andprocessed to have a desired shape, so that the source electrode 107 aand the drain electrode 107 b are formed. Then, a semi-conductive oxidefilm is formed over the source electrode 107 a and the drain electrode107 b and etched, whereby the semi-conductive oxide film 105 b having adesired shape (e.g., island shape) is obtained (see FIG. 4C).

Further, the first heat treatment described in Embodiment 1 isperformed, and in particular, the semi-conductive oxide film 105 b in aportion in contact with the semi-conductive oxide layer 104 a is made tobe crystallized, whereby the semi-conductive oxide film 105 a isobtained (see FIG. 4D). In addition, as illustrated in FIG. 4E, the gateinsulator 112 is deposited and the gate electrode 113 is formed over thegate insulator 112.

Further, the first insulator 108 is formed by a sputtering method, a CVDmethod, or the like and the second insulator 109 which has a flatsurface is formed thereover. Then, the first insulator 108 and thesecond insulator 109 are selectively etched, so that a contact holereaching the drain electrode 107 b is formed. The display electrode 110which is in contact with the drain electrode 107 b through this contacthole is formed (see FIG. 4F).

In this embodiment, the manufacturing process of the display deviceusing a transistor is described; it is apparent that the methoddisclosed in this embodiment can be applied not only to a display devicebut also an electronic device of another embodiment (e.g., an integratedcircuit).

Embodiment 5

In this embodiment, an example of a display device having a transistorwhich is different from the transistor described in any of the aboveembodiments will be described. A transistor illustrated in FIG. 5A isone of dual-gate transistors including two gate electrodes above andbelow a semiconductor layer.

The transistor includes, over the substrate 101 having an insulatingsurface, the first gate electrode 102, the first gate insulator 103, theinsulating oxide layer 104 b which includes gallium as a main metalelement, the semi-conductive oxide film 105 b, the N-typesemi-conductive oxide films 106 a and 106 b, the source electrode 107 a,the drain electrode 107 b, the second gate insulator 112, and the secondgate electrode 113. Further, the display device described in thisembodiment includes the insulator 109 having a flat surface, and thedisplay electrode 110 connected to the drain electrode 107 b through acontact hole which penetrates the insulator 109 and the second gateinsulator 112 and which reaches the drain electrode 107 b.

In the above structure, for materials, manufacturing methods, and thelike of the second gate insulator 112 and the second gate electrode 113,refer to those of the gate insulator 112 and the gate electrode 113 ofEmbodiment 3 or 4. For the other components, refer to the contentsdescribed in Embodiment 1.

When a semi-conductive oxide layer is used for a semiconductor layer ina transistor, the threshold voltage of the transistor sometimes shiftsin the positive or negative direction depending on a manufacturingprocess of a semiconductor device in some cases. Therefore, like theabove-described transistor, a dual-gate transistor in which the secondgate electrode 113 is provided over the second gate insulator 112 sothat the threshold voltage can be controlled is preferably used. Thepotential of the first gate electrode 102 or the potential of the secondgate electrode 113 is controlled, whereby the threshold voltage can bemade to be an appropriate level.

Further, the first gate electrode 102 and the second gate electrode 113can block light irradiation from the outside; thus, variation in theelectrical characteristics of the transistor due to light irradiationperformed on the semi-conductive oxide film 105 b can be suppressed.

In this embodiment, the manufacturing process of the display deviceusing a transistor is described; it is apparent that the methoddisclosed in this embodiment can be applied not only to a display devicebut also an electronic device of another embodiment (e.g., an integratedcircuit).

Embodiment 6

In this embodiment, an example of a display device having a dual-gatetransistor which is different from the dual-gate transistor described inEmbodiment 5 is illustrated in FIG. 5B. This display device includes,over the substrate 101 having an insulating surface, the first gateelectrode 102, the first gate insulator 103, the insulating oxide layer104 b which includes gallium as a main metal element, the sourceelectrode 107 a, the drain electrode 107 b, the semi-conductive oxidefilm 105 b, the second gate insulator 112, and the second gate electrode113.

Further, the display device described in this embodiment includes theinsulator 109 having a flat surface, and the display electrode 110connected to the drain electrode 107 b through a contact hole whichpenetrates the insulator 109 and the second gate insulator 112 and whichreaches the drain electrode 107 b.

In the above structure, for materials, manufacturing methods, and thelike of the second gate insulator 112 and the second gate electrode 113,refer to those of the gate insulator 112 and the gate electrode 113 ofEmbodiment 3 or 4. For the other components, refer to the contentsdescribed in Embodiment 1 or 2. The display device illustrated in FIG.5B differs from that in FIG. 5A in the positional relation between thesemi-conductive oxide film 105 b, and the source electrode 107 a and thedrain electrode 107 b.

Embodiment 7

The display device disclosed in any of Embodiments 1 to 6 can be appliedto a variety of electronic devices (including an amusement machine).Examples of electronic devices are a television set (also referred to asa television or a television receiver), a monitor of a computer or thelike, a camera such as a digital camera or a digital video camera, adigital photo frame, a mobile phone handset (also referred to as amobile phone or a mobile phone device), a portable game machine, aportable information terminal, an audio reproducing device, alarge-sized game machine such as a pachinko machine, and the like.

In addition, a transistor having the structure disclosed in any ofEmbodiments 1 to 6 can be used for, other than a display device, variousintegrated circuits (including a memory device), an electronic devicewhich incorporate the integrated circuits, an electric appliance whichincorporate the integrated circuits, and the like, for example.

This application is based on Japanese Patent Application serial no.2010-187878 filed with Japan Patent Office on Aug. 25, 2010, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A manufacturing method of a semiconductor device,comprising the steps of: forming a first film that contains an oxideincluding at least a first metal element and a second metal element,over a substrate; heating the first film to form a first layer thatcontains a crystal of an oxide including the first metal element as amain metal component and a second layer that is on a side closer to thesubstrate than the first layer and contains an oxide including thesecond metal element as a main metal component; forming a second filmthat is in contact with the first layer and contains an oxide; andheating the first layer and the second film, wherein a concentration ofsodium in at least one of the first layer and the second film is lowerthan 5×10¹⁶ cm⁻³.
 2. The manufacturing method of a semiconductor device,according to claim 1, wherein the first layer and the second filmcomprise semi-conductive oxide.
 3. The manufacturing method of asemiconductor device, according to claim 1, wherein the second filmcomprises In—Ga—Zn-based oxide semiconductor.
 4. The manufacturingmethod of a semiconductor device, according to claim 1, wherein a ratioof the second metal element to a metal element in the second film isgreater than or equal to 0.2.
 5. The manufacturing method of asemiconductor device, according to claim 1, wherein the first film isformed by a DC sputtering method.
 6. A manufacturing method of asemiconductor device, comprising the steps of: forming a first film thatcontains an oxide including at least a first metal element and a secondmetal element, over a substrate; heating the first film to form a firstlayer that contains a crystal of an oxide including the first metalelement as a main metal component and a second layer that is on a sidecloser to the substrate than the first layer and contains an oxideincluding the second metal element as a main metal component; forming asecond film that is in contact with the first layer and contains anoxide; and heating the first layer and the second film to form a thirdlayer from the first layer and the second film, wherein a concentrationof sodium in at least one of the first layer and the second film islower than 5×10¹⁶ cm⁻³, and wherein a carrier concentration of the thirdlayer is lower than 1×10¹⁴ cm⁻³.
 7. The manufacturing method of asemiconductor device, according to claim 6, wherein the first layer, thesecond film, and the third layer comprise semi-conductive oxide.
 8. Themanufacturing method of a semiconductor device, according to claim 6,wherein the second film comprises In—Ga—Zn-based oxide semiconductor. 9.The manufacturing method of a semiconductor device, according to claim6, wherein a ratio of the second metal element to a metal element in thesecond film is greater than or equal to 0.2.
 10. The manufacturingmethod of a semiconductor device, according to claim 6, wherein thefirst film is formed by a DC sputtering method.
 11. A semiconductordevice comprising: a gate electrode over a substrate; a first oxidesemiconductor layer with a gate insulating film between the gateelectrode and the first oxide semiconductor layer; a source electrodeand a drain electrode over the first oxide semiconductor layer; and asecond oxide semiconductor layer over the source electrode and the drainelectrode, wherein a part of the first oxide semiconductor layer is incontact with a part of the second oxide semiconductor layer.
 12. Thesemiconductor device, according to claim 11, wherein the first andsecond oxide semiconductor layers comprise In—Ga—Zn-based oxidesemiconductor.